The present invention is directed to a system for delivering high energy laser light by means of an optical waveguide, and in one particular application is concerned with laser angioplasty and a means for guiding such a system.
The use of laser energy to ablate atherosclerotic plaque that forms an obstruction in a blood vessel is a viable alternative to coronary bypass surgery. This procedure, known as angioplasty, essentially involves insertion of a fiberoptic waveguide catheter into the blood vessel, and conduction of laser energy through the catheter's waveguide to direct it at the plaque once the distal end of the catheter is positioned adjacent the obstruction.
Most of the early experimentation and testing that was done in this area utilized continuous wave laser energy, such as that produced by Argon Ion, Nd:YAG or Carbon Dioxide lasers. The light produced by this type of laser is at a relatively low energy level, and it can be readily conducted through an optical fiber. Ablation of the obstruction is achieved with these types of lasers by heating the plaque with constant laser power over a period of time until the temperature is great enough to destroy it.
While the use of continuous wave laser energy has been found to be sufficient to ablate an obstruction, it is not without its drawbacks. Most significantly, the destruction of the lesion is uncontrolled and is accompanied by thermal injury to the vessel walls immediately adjacent the obstruction. In an effort to avoid such thermal injury and to provide better control of the tissue removal, the use of a different, higher level form of laser energy having a wavelength in the ultra-violet range (40-400 nanometers) has been suggested. See, for example, International Patent Application PCT/US84/02000, published Jun. 20, 1985. One example of a laser for producing this higher level energy is known as the Excimer laser, which employs a laser medium such as argon-fluoride having a wavelength of 193 nanometers, krypton-chloride (222 nm), krypton-fluoride (248 nm), xenon-chloride (308 nm) or xenon-fluoride (351 nm). The light produced by this type of laser appears in short bursts or pulses that typically last in the range of tens of nanoseconds and have a high peak energy level, for example as much as 200 mJ. Although the destruction mechanism involving this form of energy is not completely understood, it has been observed that each single pulse of the Excimer laser produces an incision which destroys the target tissue without significant accompanying thermal injury to the surrounding area. This result, which is referred to as "photoablation", has been theorized to be due to either or both of two phenomena. The delivery of the short duration, high energy pulses may vaporize the material so rapidly that heat transfer to the non-irradiated adjacent tissue is minimal. Alternatively, or in addition, ultraviolet photons absorbed in the organic material might disrupt molecular bonds to remove tissue by photochemical rather than thermal mechanisms.
While the high peak energy provided by Excimer and other pulsed lasers has been shown to provide improved results with regard to the ablation of atherosclerotic plaque, the high energy and power density also presents a serious practical problem. Typically, to couple a large diameter laser beam into a smaller diameter fiber or bundle of fibers, the input ends of the fibers are ground and polished to an optical grade flat surface. Residual impurities from the polishing compound and gross scratches on the surface absorb the laser energy. These imperfections result in localized expansion at the surface of the fiber when the laser energy is absorbed. The high energy and high power Excimer laser pulses contribute to high shear stresses which destroy the integrity of the fiber surface. Continued application of the laser energy causes a deep crater to be formed inside the fiber. Thus, it is not possible to deliver a laser pulse having sufficient energy to ablate tissue in vivo using a conventional system designed for continuous wave laser energy.
This problem associated with the delivery of high-energy, high-power laser pulses is particularly exacerbated in the field of coronary angioplasty because of the small diameter optical fibers that must be used. For example, a coronary artery that is appropriate for laser angioplasty typically has an internal diameter of about three millimeters or less. Accordingly, the total external diameter of a coronary angioplasty catheter system is typically less than two and one-half millimeters. If this catheter system is composed of multiple optical fibers arranged adjacent one another, it will be appreciated that each individual fiber must be quite small in cross-sectional area.
A critical parameter with regard to the destruction of an optical fiber is the density of the energy and power that is presented to the end of the fiber. In order to successfully deliver the laser energy, the energy and power density must be maintained below the destruction threshold of the fiber. Thus, it will be appreciated that fibers having a small cross-sectional area, such as those used in angioplasty, can conduct only a limited amount of energy if the density level is maintained below the threshold value. This limited amount of energy may not be sufficient to efficiently ablate the obstructing tissue or plaque without thermal damage.
Even if the energy density is quite high, the small beam that results from a small diameter fiber may not have a sufficiently large target area that effective ablation of the lesion results. Only a small fragment of the lesion might be ablated, and thus not provide adequate relief from the blockage.
A further problem with the use of a fiberoptic waveguide catheter to direct laser energy for purposes of ablating atherosclerotic plaque is that of perforation of the blood vessel. Such perforations can be caused by the catheter itself contacting and mechanically perforating the vessel. Such perforations can also be caused by the laser beam, particularly if the catheter is not aligned properly within the blood vessel. The perforation problems are related to the intrinsic stiffness of the glass fibers of the waveguide catheter and poor control of laser energy at the distal end of the fibers, regardless of laser source or wavelength.
Also related to the stiffness of the glass fibers is the ability to control the position of the laser-conducting fibers radially within the blood vessels. The conventional systems that employ a fiberoptic waveguide catheter within a blood vessel do not provide means for controlling radial movement within the blood vessel.
One known attempt at developing an angioplasty catheter is disclosed in U.S. Pat. No. 4,747,405. The known catheter includes a center guidewire lumen, a guidewire therein, and a single optical fiber disposed at a side of the catheter for emitting laser energy. The catheter also has a blunt leading end that does not facilitate progress through a blood vessel. A particular problem that potentially results from the use of a catheter with a single optical fiber is that large segments of the ablated lesion may become loose in the blood stream and could possibly cause an emboli. As a result, the known catheter includes a dedicated channel to remove the loosened debris.